I tried out postmarketOS + phosh on a PinePhone about a year ago. For my own needs, it worked fairly well, except (ironically) receiving calls. It was like driving an old car, everything was slightly jank, but worked, and could be tinkered with - see the entire review. I have to give credit that there has been impressive progress in mobile Linux since PinePhone's release in 2019, and a lot of it was developed by unpaid hobbyists.

I don't daily drive one but I've been keeping a close eye on it and here's my opinions: postmarketos seems to have the most momentum out of any distro (you can see device support here). I do believe it will be viable eventually as a lot of work is actively being done. This month they announced two grants that were accepted for 4g volte calling and firefox on mobile improvements. They are also working on porting systemd to alpine so that gnome mobile and plasma mobile can be run without any workarounds. Also the oneplus6/6t seems to be the most hopeful for a daily driver.

tldr: I don't think it's currently viable but work and money is currently being put towards projects to fix that

It depends. It's viable if you just need a phone with several open source applications (non-Android) and are fine with that. But if you need Android app compatibility it's probably going to be harder or more inconvenient to do, though I haven't checked the status in recent time. And then there's this evil thing called Google Play Integrity (essentially DRM restricting which apps can run on which OS) which is a problem even for non-proprietary Androids, so you probably won't have any chance if you're dependent on such an app (thankfully it's rare but as we all know stupid ideas tend to become annoyingly popular).

Main problem, as usual, is that Android and iOS have become such big and popular "platforms" for mobile apps that establishing a "third" platform for app developers is basically impossible (also remember what happened to Windows Phone OS, they were late to the market and failed spectacularly to catch up. Of course in this case it's open source so it can grow regardless of user numbers, but still, it's hard to catch up when lots of great Android apps were already developed specifically for Android). So you can only hope that Android app compatibility grows mature enough to be close to 100% compatible, so that you can also run almost all Android apps on your mainline Linux mobile OS. Then you're not "limited" anymore. (At least if you consider it "limited" when you can't run Android apps. Which most probably consider to be "limited").

So I think it's less about the hardware and OS/UI (I think they work fine these days) and more about the available apps.

[My main daily driver phone is a GrapheneOS (Android) and I have a Pinephone with Linux for playing around in WiFi at home only]

Gonna pull your leg here and say Android or, as I've recently taken to calling it, busybox + Linux + Google

No bc of camera proprietariness

I'm surprised nobody mentioned Droidian yet. It's the best of both worlds: You get a Linux phone with Phosh and an actual camera + sensors working due to the Android kernel. Check it out here: https://droidian.org/

It supports Waydroid out of the box, allowing you to run Android apps such as Whatsapp, Bitwarden and even Google Playstore, etc.

The new Firefox is miles away from what PostmarketOS offers. The only downside is you need a supported device, as per https://devices.droidian.org/.

So yes, I do drive Droidian daily, but I have an Android phone nearby just in case I need something specific.

SailfishX is a usable daily driver with decent Android app support.

BUT: you’ll have to be okay with dealing with random annoyances like:

-> Your default weather app lost the ability to get weather data.

-> Some Xperia 10 III devices lose audio after some time when using GPS. Unless you are in Finland, then you are fine. Nobody knows why.

fwiw: it was viable when i had the first android released to the public; it was an HTC and with debian.

I use postmarketos with phosh. It's kind of viable, but it has some infrequent bugs. For example sometimes, quite rarely, the call menu may freeze after the call and not respond to touches until the reboot. The camera doesn't work at all. But there are positive aspects, an ordinary Linux terminal and the usual convenient console programs. :)

I run Ubuntu Touch on a Google Pixel 3a xL with Waydroid/Aurora store for Android stuff. So far it works fine for what i need.

The biggest hurdle is getting a phone that you even can install a custom ROM or different OS. 'mericans and yuropeans can get their pixels, pinephones and similars easily, other places cannot.

How are all the AOSP-based OSes, like for instance GrapheneOS, not Linux distros?

tried pmos with plasma mobile but the browsers were buggy because of my SOC having bad graphics acceleration support in the mainline kernel

Does android count?

...does android count

No, none of them are. I tried 2-3 versions of it, none is good. However, Android on the other hand, which is also linux-based, is good. Go for Murena's e/OS, or LineageOS.

No. Maybe. Why not?

Android is a Linux distro, just because its not gnu or running whatever subset of features a desktop Linux might have doesn't make it any less of a Linux distro.

The real question is what do you consider a part of a "Linux disto" that currently isn't available on android?